Electrokinetic flow and transport
“Digital” microfluidics means that small portions of liquid stay in a confined space when being processed without suffering from hydrodynamic dispersion or other detrimental effects that dilute the sample. The most popular realization of digital microfluidics relies on small droplets that are transported and processed, for example using electrowetting-on-dielectric. We propose an alternative route towards digital microfluidics which is based on isotachophoretic sample transport. In isotachophoresis (ITP) samples are separated and stacked according to their electrophoretic mobilities by sandwiching them between two buffers of high and low mobilities and applying an electric field. This mechanism allows us to create and to manipulate small sample plugs (approx. 10 µm in length). The usual operations of droplet-based digital microfluidics, namely creating, transporting, merging and splitting samples, all find their counterparts in ITP-based microfluidics.
The figure below shows how two ITP samples are brought into contact in a microchannel. First, one of the samples is transported into the exit channel (upper left), then the other sample follows (lower left). Within the exit channel the second sample catches up and overruns the first one (right side). During this process chemical reactions between the samples such as DNA hybridizations can be carried out, as we have shown.
ITP can also be used for the transport and separation of microparticles. Performing ITP only with a leading and a terminating electrolyte results in a sharp transition zone between the two electrolytes. This transition zone can carry microparticles along with it like a conveyer belt. The three snapshots in the figure below demonstrate the collection and transport of 5 µm polymer beads by the ITP transition zone. We have shown that the transport mechanism is size specific: While larger beads are picked up by the transition zone, smaller beads are not. This allows a size separation of microparticles.
G. Goet, T. Baier and S. Hardt, Transport and separation of micron sized particles at isotachophoretic transition zones, Biomicrofluidics 5 (2011), 014109.
G. Goet, T. Baier and S. Hardt, Micro contactor based on isotachophoretic sample transport, Lab Chip 9 (2009), 3586-3593.
Electroosmotic flow along superhydrophobic surfaces
A superhydrophobic surface exhibits nano- or microscale topographical features. In the Cassie state the liquid wetting the surfaces does not penetrate into the indentations, leaving them filled with gas. Such surfaces do exhibit a number of favorable features, such as reduced hydrodynamic drag. However, it has been shown that under reasonable assumptions the electroosmotic flow (EOF) along a superhydrophobic surface is not enhanced compared to the flow along its flat, unstructured counterpart. This can be explained by studying the charge structure of the gas-liquid interfaces. Either these interfaces are not charged, or they carry mobile ions that are screened by counter charges (see the figure below, left part). In any case, the gas-liquid interfaces are not able to contribute to the EOF enhancement. The situation changes if a net charge at the gas-liquid interfaces is created artificially by employing auxiliary electrode structures, as shown at the right of the figure below. In that case a net charge at those interfaces is created, giving rise to a body force in a region of low flow resistance. We predict that in such a way we should be able to enhance the EOF by more than two orders of magnitude compared to a flat surface. A limiting factor to this mechanism is the Cassie-to-Wenzel transition at the superhydrophobic surface. In any case, this scheme promises novel pathways towards highly dynamic electrically operated fluidic actuators.
C. Schönecker and S. Hardt, Electro-osmotic flow along superhydrophobic surfaces with embedded electrodes, Phys. Rev. E 89 (2014), 063005.